T2345: After examining comments changed by batch conversion of NULL

to nullptr, I've corrected a small number where it seemed clear
that the previous NULL should have been either "NULL" (where the
code is looking for a string with that value),  or null char / null byte
(where the comment is talking about the char/byte that terminates
a c-style string).

Also changed 6 places where the batch change had changed 'NULL ptr'
to 'nullptr ptr'. Now they simply say 'nullptr'.